No need to enter a scrutinised value

While analysing the output of #13861 I stumbled over an unnecessary pessimisation in handling of scrutinised values. With words of Simon (from https://phabricator.haskell.org/D4267 with minor edits added):

Interesting. Yes, please make a ticket! (And transfer the info below into it.)

I think the issue is this. Given (the STG-ish code)

data Colour = Red | Green | Blue
f x = case x of y
           Red -> Green
           DEFAULT -> y

(here y is the case binder) we can just return x rather than entering it in DEFAULT branch, because y will be fully evaluated and its pointer will be correctly tagged.

You absolutely can't check for an OccName of "wild"!! That is neither necessary nor sufficient :-).

Instead, check isEvaldUnfolding (idUnfolding y). See Note [Preserve evaluatedness] in CoreTidy.hs. And be sure to augment that note if you make the change.

I would expect perf benefits to be small on average, but it's simple to implement.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related	#13861
Blocking
CC
Operating system
Architecture